1. Field of the Invention
The invention relates to flux converters and, more particularly, to a flux converter for converting an input-side AC voltage into an output-side DC voltage to provide a power factor correction. The invention furthermore relates to a method for operating such a flux converter.
2. Description of the Related Art
Generally, converters used in normal network power supplies exhibit input currents having significant harmonics or a power factor well below one. The reason for this lies in the requirement to recharge a storage capacitor on the DC side by a two-way rectifier. Short, needle-shaped current peaks are the result. Without additional measures, the level of these current peaks is limited only by the internal resistances of an input-side network, a network filter, a rectifier and a storage capacitor.
Power supplies on a single-phase power network, i.e., those having fairly high through-ratings (e.g., 200 W and above), require a special design to meet applicable technical specifications. These design measures can comprise a considerable proportion of the cost of a power supply. In addition, the overall size and the degree of efficiency are negatively influenced as a consequence of additional power losses.
In order to reduce a proportion of interference harmonics with regard to the current draw of a converter, a power factor correction (PFC) is provided in accordance with the prior art.
A passive power factor correction is achieved by a high input inductance. A high inductance value is necessary to substantially increase the conduction angle during a recharging phase. This only makes sense in the case of low power outputs because otherwise the corresponding chokes are too large and heavy. In addition to the costs of the chokes, their power loss must be taken into consideration.
This conventional method is not well suited for wide input voltage ranges on account of the variance in the maximum input current associated therewith.
Alternatively, it is known to use an active power factor correction, where a separate converter stage readjusts the current drawn to the timing profile of a sinusoidal network voltage. Generally, such active PFC circuits are designed as step-up converters and are connected directly downstream of a rectifier. These step-up converters charge a large capacitor up to a voltage in excess of the peak voltage of the AC input voltage. The step-up converter operates at a significantly higher frequency than a network supply, which means that a considerably smaller inductance is required. An almost continuous current flow having a low current ripple is produced, whereby the average current is adjusted by a control circuit to the instantaneous value of the network voltage.
Compared with a passive power factor correction, although an active PFC circuit is more complex, greater degrees of efficiency and an improved suppression of harmonics are however possible. In addition to the complexity, a disadvantage is the output voltage of such a PFC circuit, which in principle exceeds the maximum network voltage, as a result whereof problems can occur primarily with regard to high network input voltages in respect of component loadings and insulation voltages.
Instead of a step-up converter, an active PFC circuit can comprise a step-down converter, at which an output voltage less than the network voltage is present. The possible conduction angle decreases as a result, however. The energy input into a storage capacitor can only occur with a network voltage greater than the voltage at the storage capacitor. In addition, the current ripple is higher than in the case of a solution with a step-up converter and the activation of a power switch on the ground side is simpler with a step-up converter.
Active PFC circuits have the disadvantage that they must be provided in addition to the actual converter. Compared with converters not having PFC functionality, this means significant additional expenditure and additional losses.